Films are commonly used as barriers. Some films are engineered to be liquid impervious and moisture impervious such as those used as protective packages or those used in absorbent products. Other films are engineered to be liquid impervious and moisture permeable such as those used in diapers or adult incontinence products.
In some instances, it may be desirable for the product or package to not produce a lot of noises. It is desirable for products such as feminine pads, incontinence products, tampons, and their packaging to be discrete, therefore it is preferred that these products and packages generate the lowest amount of noise possible. Other products like diapers and premium packages seek to mimic the appearance and feel of cloth, therefore plastic sounding materials are less desirable.
Sound is the result of a vibrating object pushing the air surrounding it and creating bands of high and low pressure. These bands of high and low pressure are longitudinal waves that the human ear perceives as sound. Each sound can be described by a magnitude, known as sound pressure level measured in decibels (dB) and a frequency which is measured in hertz (Hz). The human auditory system is most sensitive from 2,000 to 5,000 Hz due to the resonance of the ear canal. An object vibrates as a result of an input force of a certain magnitude and frequency and the mass, stiffness and damping characteristics of the object. One way to reduce the noise generated by a product containing a film is to reduce the noise produced by the film. This can be achieved by reducing the film stiffness, increasing its mass or changing its damping characteristics.
The stiffness of an elastic film is calculated using the Young's modulus E of the film. The Young's modulus is the slope of the stress strain curve at the strains of interest. Typically, films are anisotropic, therefore they will have different Young's modulus depending on the tensile test direction. The speed at which the tensile test is conducted will also impact the resulting modulus. The measurements should be taken at speeds relevant to the application.
The strain levels that vibrating thin polyolefin films experience are small and in most cases below the yield point of the film. Therefore the strains considered when calculating Young's modulus are between 0.01 and 0.05.
The extrusion and processing conditions under which a film is produced will impact the level of crystallization on the film which in turn will affect the resulting modulus and noise produced by the film. For example, in cast extrusion processes the temperature of the casting roll, chill rolls temperatures and arrangements, melt temperature, take off-speed and annealing roll temperature will affect the level of crystallization on the film and may impact the film stiffness and noise.
Polymer structures can be anywhere from 5% to 95% crystalline. In addition to extrusion processing conditions, the level of crystallinity will also depend on the simplicity of the chain structure, chemistry, side branching and whether the polymer is isotactic, syndiotactic or atactic. Some of the known methods used to measure the amount of crystallinity in a polymer make use of Differential Scanning calorimetry, X-Ray Diffraction or by a density measurement if the density of the crystal phase and amorphous phases are known. The more crystalline the polymer structure, the higher the Young's modulus and the noise produced by the resulting film. For example: Resin blends with higher content of resins with higher side branching such as Low Density Polyethylene are preferred as their typical degree of crystallinity is 45-55%. Less content of polymers with chain regularity is preferred such as isotactic polypropylene which typical degree of crystallinity is 70%-80%.
Incremental stretching of thermoplastic film typically involves running the film between grooved or toothed rollers. The grooves or teeth on the rollers intermesh and stretch the film as the film passes between the rollers. Incremental stretch can stretch a film in many small increments that are evenly spaced across the film. The depth at which the intermeshing teeth engage can control the degree of stretching. One type of incremental stretching is referred to as ring-rolling.
Therefore, it is an object of the present invention to produce films that have a sound pressure level of less than about 43 dB as determined by the Sound Pressure Level Test herein.